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| From | Anton Shepelev <anton.txt@gmail.com> |
|---|---|
| Newsgroups | comp.lang.c |
| Subject | Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] |
| Date | 2016-11-30 00:14 +0300 |
| Organization | A noiseless patient Spider |
| Message-ID | <20161130001448.0acf489caa3aa31becdd5e2c@gmail.com> (permalink) |
| References | <87oa1zgzw0.fsf@starless.ames.family> <20161121165927.aae89b9f861247b4a268e1bd@g{oogle}mail.com> <69c8a6e9-9685-6047-21fd-0721bcf899a1@verizon.net> <20161122232309.40aa1289366c35d048c594f3@gmail.com> <2985f44b-b799-f6bb-142e-f2c1eb9d7ee2@verizon.net> |
James R. Kuyper to Anton Shepelev:
> > The author makes extensive use if casting be-
> > tween non-aliasing pointers. He author writes:
> >
> > 1. Passing a circle as a point means convert-
> > ing from a struct Circle * to a
> > struct Point *. We will refer to this as
> > an up-cast from a subclass to a super-
> > class -- in ANSI-C it can only be accom-
> > plished with an explicit conversion opera-
> > tor or through intermediate void * values.
> >
> On page 34, he writes:
> struct Circle { const struct Point _; int rad; };
>
> Given
>
> struct Circle c, *pc=&c;
> struct Point p, *pp=&p;
>
> The explicit conversion he referred to is
> (const struct Point *)pc ,
> and it is allowed by 6.7.2.1p15.
Is it the item saying that a pointer suitably con-
verted shall point the first field of a struct? I
am sorry I have no standard at hand.
> However, he's wrong about void* being the only
> other way to do this. &pc->_ is simpler and equiv-
> alent, and will continue to work even if some oth-
> er field becomes the first member of struct Cir-
> cle.
In OOP this method is out of the question, because a
member should be accessible immediately from the ob-
ject at hand, without knowing the inheritance level
at which it has been introduced. Whereas the ->_ in
your example betrays the object's internal structure
and the assumption that it is derived from Point.
What should you do if there were a three-tier hier-
archy -- ->_->_? For aught I know, this is allowed
under the hood but not in user code.
> > 2. It is usually unsound, however, to pass a
> > point to a function intended for circles
> > such as Circle_draw(): converting from a
> > struct Point * to a struct Circle * is on-
> > ly permissible if the point originally was
> > a circle. We will refer to this as a
> > down-cast from a superclass to a sub-
> > class -- this requires explicit conver-
> > sions or void * values, too, and it can
> > only be done to pointers to objects that
> > were in the subclass to begin with.
> >
> In other words, given
>
> pp = &c._;
>
> then (struct Circle*)pp is also allowed by
> 6.7.2.1p15. However, given:
>
> pp - &p;
>
> then (struct Circle*)pp would not be allowed.
>
> I don't see any problem with this approach. What
> are your concerns about it?
The problem is that the author uses the latter ex-
pression. In the appendix "ANSI-C Programming
Hints" he shows stark innocence of the aliasing lim-
itations:
The first component of a structure starts right
at the beginning of the structure; therefore,
structures can be lengthened or shortened:
struct a { int x; };
struct c { struct a a; ... } c, * cp = & c;
struct a * ap = & c.a;
ANSI-C permits neither implicit conversions of
pointers to different structures nor direct
access to the components of an inner structure:
cp -> a.x ok, fully specified
((struct a *) cp) -> x ok, explicit conversion (!!!)
Or am missing something?
--
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Aliasing and subtyping structs Andreas <nospam@invalid.invalid> - 2016-11-01 13:15 +0100
Re: Aliasing and subtyping structs Andreas <nospam@invalid.invalid> - 2016-11-01 13:34 +0100
Re: Aliasing and subtyping structs Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-11-01 15:27 +0000
Re: Aliasing and subtyping structs Andreas <nospam@invalid.invalid> - 2016-11-02 09:47 +0100
Re: Aliasing and subtyping structs Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-11-02 15:47 +0000
Re: Aliasing and subtyping structs Andreas <nospam@invalid.invalid> - 2016-11-08 00:24 +0100
Re: Aliasing and subtyping structs supercat@casperkitty.com - 2016-11-07 21:42 -0800
Re: Aliasing and subtyping structs <william@wilbur.25thandClement.com> - 2016-11-03 18:53 -0700
Re: Aliasing and subtyping structs supercat@casperkitty.com - 2016-11-04 09:56 -0700
Re: Aliasing and subtyping structs Andreas <nospam@invalid.invalid> - 2016-11-08 21:53 +0100
Re: Aliasing and subtyping structs James Kuyper <jameskuyper@verizon.net> - 2016-11-01 11:41 -0400
Re: Aliasing and subtyping structs Andreas <nospam@invalid.invalid> - 2016-11-02 09:55 +0100
"Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Anton Shepelev <anton.txt@g{oogle}mail.com> - 2016-11-21 16:59 +0300
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-21 07:32 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "James R. Kuyper" <jameskuyper@verizon.net> - 2016-11-21 11:15 -0500
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-21 08:35 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Anton Shepelev <anton.txt@g{oogle}mail.com> - 2016-11-22 11:42 +0300
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-22 06:50 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-11-22 16:07 +0000
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-22 12:02 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Ben Bacarisse <ben.usenet@bsb.me.uk> - 2016-11-22 23:36 +0000
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-22 16:24 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Richard Damon <Richard@Damon-Family.org> - 2016-11-22 22:21 -0500
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-23 08:31 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] me <crisdunbar@gmail.com> - 2016-11-23 09:01 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-23 12:05 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Anton Shepelev <anton.txt@gmail.com> - 2016-11-22 23:23 +0300
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "James R. Kuyper" <jameskuyper@verizon.net> - 2016-11-22 16:02 -0500
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-22 14:13 -0800
Re: container_of() macro Noob <root@127.0.0.1> - 2016-11-23 01:10 +0100
Re: container_of() macro <william@wilbur.25thandClement.com> - 2016-11-22 20:10 -0800
Re: container_of() macro "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-11-23 15:10 -0800
Re: container_of() macro David Brown <david.brown@hesbynett.no> - 2016-11-23 11:31 +0100
Re: container_of() macro Anton Shepelev <anton.txt@g{oogle}mail.com> - 2016-11-23 14:07 +0300
Re: container_of() macro supercat@casperkitty.com - 2016-11-23 08:41 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Anton Shepelev <anton.txt@gmail.com> - 2016-11-30 00:14 +0300
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "James R. Kuyper" <jameskuyper@verizon.net> - 2016-11-29 17:23 -0500
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Anton Shepelev <anton.txt@gmail.com> - 2016-12-04 18:48 +0300
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "James R. Kuyper" <jameskuyper@verizon.net> - 2016-12-04 12:17 -0500
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Anton Shepelev <anton.txt@gmail.com> - 2016-12-04 22:15 +0300
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "James R. Kuyper" <jameskuyper@verizon.net> - 2016-12-04 15:06 -0500
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Anton Shepelev <anton.txt@gmail.com> - 2016-12-12 22:31 +0300
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-11-23 13:39 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] luser droog <luser.droog@gmail.com> - 2016-11-23 15:10 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-11-23 15:13 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] luser droog <luser.droog@gmail.com> - 2016-11-23 18:29 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Anton Shepelev <anton.txt@g{oogle}mail.com> - 2016-11-24 11:41 +0300
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Thiago Adams <thiago.adams@gmail.com> - 2016-11-24 03:29 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-11-24 17:37 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-25 09:55 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Thiago Adams <thiago.adams@gmail.com> - 2016-11-25 15:22 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-11-28 18:18 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Thiago Adams <thiago.adams@gmail.com> - 2016-11-29 04:48 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-29 07:28 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Thiago Adams <thiago.adams@gmail.com> - 2016-11-30 08:26 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] supercat@casperkitty.com - 2016-11-30 09:13 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] "Chris M. Thomasson" <invalid@invalid.invalid> - 2016-12-12 16:44 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Melzzzzz <mel@zzzzz.com> - 2016-12-13 02:18 +0100
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Thiago Adams <thiago.adams@gmail.com> - 2016-12-14 09:47 -0800
Re: "Object-oriented programming in C" [was: Re: Aliasing and subtyping structs] Thiago Adams <thiago.adams@gmail.com> - 2016-12-14 10:14 -0800
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